This invention relates to aromatic amines, more particularly to N-cyanoethylated ortho-toluenediamine and N-cyanoethylated meta-toluenediamine, a process for making them, and their use as epoxy curing agents.
It is known to use amines, such as aliphatic or aromatic amines, for the curing of epoxy resins. See, e.g., xe2x80x9cHandbook of Epoxy Resinsxe2x80x9d by H. Lee and K. Neville, McGraw Hill Book Co., 1967. The aromatic amines currently available provide high Tg, good fracture toughness and excellent chemical resistance for adhesive and composite applications, but suffer from several shortfalls. Aromatic amines typically used as epoxy curatives, such as methylenedianiline (MDA) and diethyltoluenediamine (DETDA), are usually considered to be highly toxic. In addition, the potlife of such aromatic amines is of an insufficient duration to consider the amines to be latent curatives. MDA and DETDA, for example, have very long potlives of up to 12 to 24 hours, but do not provide a potlife of several days, which is desired in a latent curing agent. Moreover, such aromatic amines cannot be used with solid epoxy resins for powder coating applications.
The epoxy industry has employed many types of curative blends in an attempt to maximize the desired application properties, but in most cases at the expense of other properties. Additives such as accelerators, tougheners, reactive diluents and non-reactive diluents are employed to maximize a desired property but again to the deterioration of other properties. A number of good references are available on this subject including: Lee and Neville""s, xe2x80x9cHandbook of Epoxy Resins,xe2x80x9d cited above, and W. R. Ashcroft, xe2x80x9cCuring Agents for Epoxy Resins,xe2x80x9d in B. Ellis (ed.). xe2x80x9cChemistry and Technology of Epoxy Resins,xe2x80x9d Blackie Academic and Professional, London (1993), pp. 37-73.
Others in the epoxy industry have developed novel amines in attempting to optimize curative properties. For example, Japanese Patent 2963739 (1999) describes the use of substituted-N-phenyl-1,3-propanediamines as liquid epoxy curatives which do not B-stage during cure and thus yield a fully cured epoxy resin. The substituted-N-phenyl-1,3-propanediamines described therein are represented by the following chemical formula: 
where R is hydrogen, lower alkyl group, lower alkoxyl group or halogen. Although the method of synthesis of these curatives is not disclosed, other references teach methods for synthesizing aromatic amines.
For example, European Patent 0 067 593 (1982) describes the cyanoethylation of para, meta, and ortho phenylenediamine to generate 3,3xe2x80x2-(p,m or o-phenylenedi-imino)-dipropanenitrile: 
EP 0 067 593 teaches the use of water as the solvent and concentrated hydrochloric acid as the catalyst to obtain the dicyanoethylated product.
Elderfield et al., 68 J. Amer. Chem. Soc. 1262 (1949), describes the synthesis of xcex2-p-anisidinopropionitrile by boiling p-anisidine and acrylonitrile in acetic acid.
Cookson et al., xe2x80x9cThe Cyanoethylation of Amines and Arsines,xe2x80x9d J. Chem. Soc. 1949, pp. 67-72, describes the cyanoethylation of aniline by heating to 150xc2x0 C. a mixture of aniline and acrylonitrile in the presence of excess acetic acid in an autoclave to generate 2-cyanoethylaniline and bis-2-cyanoethylaniline. The reference further describes the reaction of diphenylamine and acrylonitrile in an excess of acetic acid using a catalytic amount of fine copper powder to generate diphenyl-2-cyanoethylamine.
Braunholtz et al., xe2x80x9cThe Preparation of Bis(2-cyanoethyl) Derivatives of Aromatic Primary Amines, and Their Conversion into 1:6-Diketojulolidines,xe2x80x9d J. Chem. Soc., 1952, pp. 3046-3051, describes the cyanoethylation of aniline, m-toluidine, p-toluidine, p-anisidine and p-chloroaniline in an excess of acetic acid.
Braunholtz et al., xe2x80x9cThe Preparation of Bis(2-cyanoethyl) Derivatives of Aromatic Primary Amines, and Their Conversion into 1:6-Diketojulolidines. Part II,xe2x80x9d J. Chem. Soc., 1953, pp. 1817-1824, describes the cyanoethylation of several different aromatic primary monoamines in an excess of acetic acid using various metal catalysts to selectively generate mono and di-cyanoethylated derivatives.
All references cited herein are incorporated herein by reference in their entireties.
The invention provides cyanoethylated toluenediamines, processes for synthesizing them, compositions containing them and methods for using them to cure epoxy resins. In preferred embodiments, the cyanoethylated toluenediamines are represented by the following formula: 
where the nitrogen atoms are ortho or meta to each other on the aromatic ring.
With regard to the present invention and throughout the specification and claims the terms xe2x80x9ccyanoethyl toluenediamine(s)xe2x80x9d, xe2x80x9ccyanoethylated toluenediamine(s)xe2x80x9d, xe2x80x9cN-(cyanoethyl) toluenediamine(s)xe2x80x9d and xe2x80x9cN-(cyanoethylated) toluenediamine(s)xe2x80x9d are used interchangeably.
The most preferred cyanoethylated toluenediamines of the invention are suitable for use as epoxy resin curing agents. Cyanoethylated toluenediamines where the nitrogen atoms are ortho or meta to each other on the aromatic ring have been found to be particularly suitable for this purpose. Thus, the most preferred cyanoethylated toluenediamines of the invention are cyanoethylated products of ortho-toluenediamine represented by the following Formulas I-IV: 
and cyanoethylated products of meta-toluenediamine represented by the following Formulas V-VII: 
Formulas I-IV above are the cyanoethylated products of ortho-toluenediamine (OTD). Formulas I-II are based on 2,3-toluenediamine (TDA) and Formulas III-IV are based on 3,4-TDA. A commercial isomer mix of OTD is typically 60/40 2,3-TDA/3,4-TDA. Using commercial grade OTD therefore leads to cyanoethylated isomer mixtures.
Formulas V-VII above are the cyanoethylated products of meta-toluenediamine (MTD). Formula V is based on 2,6-TDA and Formulas VI and VII are based on 2,4-TDA.
Cyanoethylated products of OTD are prepared by reacting OTD with acrylonitrile (ACN) at elevated temperature in the presence of an acid and a protic solvent (e.g., water) for a period of time adequate for reaching the desired extent of conversion to the cyanoethylated product. Cyanoethylated products of MTD are prepared by reacting MTD with ACN at elevated temperature in the presence of an acid and a protic solvent (e.g., water) for a period of time adequate for reaching the desired extent of conversion to the cyanoethylated product. The following equation illustrates a preferred cyanoethylation reaction of the invention. 
In the preparation of cyanoethylated OTD, either the pure compounds 2,3-TDA or 3,4-TDA, or isomer mixtures, such as 60/40 2,3-TDA/3,4-TDA, can be reacted with ACN. In fact, any blend or mixture of the TDA isomers may be used. The preferred reactant mixture comprises commercial grade OTD.
In the preparation of cyanoethylated MTD, either the pure compounds 2,4-TDA, 2,6-TDA, or isomer mixtures such as 80/20 2,4-TDA/2,6-TDA can be reacted with ACN. Any blend or mixture of the TDA isomers can be used. The preferred reactant mixture comprises commercial grade MTD.
The molar ratio of reactants, moles of OTD and/or MTD to moles of ACN, can vary from about 10:1 to about 1:10. The molar ratio used will affect the rate of reaction and the product distribution, but as the final product is purified, the final product quality is unaffected. To maximize yield and efficiency, the desired molar ratio is from 0.95:1.0 to about 1.0:2.0 with the optimum being about 1.0:1.2.
The cyanoethylation reaction is conducted using at least one acid catalyst. The acid catalyst can be any mineral, carboxylic, super or supported acid, including but not limited to hydrochloric acid, sulfuric acid, phosphoric acid, acetic acid, propionic acid, para-toluenesulfonic acid, triflic acid (trifluoromethanesulfonic acid) and Nafion(copyright) super acid catalyst from DuPont, which is a bead-form strongly acidic resin, i.e., a copolymer of tetrafluoroethylene and perfluoro-3,6-dioxa-4-methyl-7-octenesulfonyl fluoride, converted to the proton (H+) form. The preferred acid catalysts are hydrochloric acid and Nafion catalyst with Nafion catalyst being most preferred.
Any protic solvent can be used in the cyanoethylation, including but not limited to water, methanol, ethanol, isopropanol, n-propanol, etc.
The cyanoethylation reaction can be conducted over a temperature range from about 50xc2x0 C. to about 150xc2x0 C. within a pressure range of from about atmospheric pressure up to about 900 psi (6.21 MPa). The reaction time is dependent on the reaction temperature, pressure and the desired extent of the reaction as measured by gas chromatography (GC). The reaction of TDA with ACN can generate not only a mono-cyanoethylated product (CNTDA) but also a di-cyanoethylated product (DCNTDA). Thus, at any given time during the cyanoethylation there is, for example, a mixture of unreacted TDA, CNTDA and DCNTDA, as described according to the following Equation V: 
The cyanoethylated product mixture can be hydrogenated to provide aminopropylated TDAs, which are also useful as epoxy curing agents. Such aminopropylated TDAs and a process for making them, are disclosed in the inventors"" copending U.S. application Ser. No. 10/024,974 also filed Dec. 19, 2001 U.S. Pat. No. 6,441,238.
The crude product resulting from cyanoethylation is a slurry mixture of starting TDA, the desired mono-cyanoethylated TDA (in the case of OTD, cyanoethylated OTD or CNOTD, and in the case of MTD, cyanoethylated MTD or CNMTD), di-cyanoethylated TDA byproduct (DCNOTD and/or DCNMTD), water and catalyst. The composition of the organic portion of the crude slurry product will vary depending on the reaction conditions, catalyst type and reaction time. Typical composition ranges for OTD-based reactions are 20/70/10 to 5/75/20 weight % OTD/CNOTD/DCNOTD. Typical composition ranges for MTD-based reactions are 20/70/10 to 5/75/20 weight % OTD/CNMTD/DCNMTD. Although the crude product is satisfactory as a curative for epoxy resins, it is more desirable to use the purified CNOTD and/or CNMTD. This purification may be accomplished by any method known to one skilled in the art, but the preferable method is simply washing the crude cyanoethylated TDA with a solvent, such as methanol, ethanol, propanol or isopropanol, again a method well known to the art. Very high purity cyanoethylated TDA can be obtained by recrystallization of the crude cyanoethylated TDA in an appropriate solvent, such as those listed above for washing the crude cyanoethylated TDA (although the washing and recrystallization solvents need not be identical). Depending on the washing or recrystallization conditions, the purified product can have an assay ranging from about 90 to 99.9% with the chief impurities being TDA and di-cyanoethylated TDA byproduct, with a small amount of acid catalyst. Again, none of these impurities will impair the performance attributes imparted by this curative to a cured epoxy resin formulation.
The crude, purified and high purity grades of cyanoethylated toluenediamine are mixed thoroughly with an epoxy resin and heated to effect curing. The stoichiometry employed preferably ranges from 0.5 Nxe2x80x94H equivalent of the CNTDA per equivalent of epoxide moieties in the epoxy resin to 1.5 Nxe2x80x94H equivalent of the CNTDA per equivalent of epoxide. More preferably, the stoichiometry ranges from 0.9 to 1.1 Nxe2x80x94H equivalent per epoxide with a 1.0:1.0 ratio being most preferred.
Any epoxy resin can be employed, including but not limited to the diglycidyl ether of bisphenol A and/or bisphenol F. Solvents such as an alcohol, phenol, aliphatic or aromatic hydrocarbon, esters, ethers and the like can be used. Reactive diluents such as aromatic and aliphatic glycidyl ethers and esters can also be used as well as various types of fillers and colorants.
The cyanoethylated aromatic amines of the invention are excellent curatives for epoxy resins, providing both latency and the performance properties of an aromatic amine curative, particularly chemical resistance and Tg. In addition, the amines of the invention are solids at room temperature.
The invention will be illustrated in more detail with reference to the following Examples, but it should be understood that the present invention is not deemed to be limited thereto.